Cucurbit downy mildew is a serious problem

Friday

Jun 30, 2017 at 11:33 AMJun 30, 2017 at 11:33 AM

Cucurbit downy mildew is a serious disease of cucurbit crops such as cucumber, watermelon, cantaloupe, squash and pumpkin. While the name “downy mildew” suggests a fungal pathogen of some sort, this disease is actually caused by a “fungus-like” oomycete, Pseudoperonospora cubensis.

Symptoms of cucurbit downy mildew include angular chlorotic lesions on the leaves, often in various shades of green to light green or yellow green. As the infection progresses, lesions can turn necrotic and merge into larger areas. In addition to the lesions, you may notice a grayish-brown “downy” appearance on the lower leaf surface.

The pathogen cannot overwinter outdoors in North Carolina, but airborne spores can ride the winds from state to state, generally starting in Florida and arriving in North Carolina sometime in early June. Commercial growers monitor the movement of the pathogen through tools such as the Cucurbit Downy Mildew Forecast Homepage (http://cdm.ipmpipe.org/index.php), and then begin fungicide sprays before it becomes established in their fields.

Control options are available for the home garden, but it will be a tough battle. According to the N.C. State factsheet “Cucurbit Downy Mildew” (https://content.ces.ncsu.edu/cucurbit-downy-mildew), most chemicals available to the home gardener are not sufficient to control cucurbit downy mildew once it appears. Products containing copper or chlorothalonil are the only effective options, and weekly sprays would be needed. And to clarify, that’s weekly sprays to maybe just barely stay one step ahead of the pathogen. Hardly seems worth it.

In the home garden, if you see your cucumber foliage beginning to decline from downy mildew, powdery mildew, or some unknown cause by early to mid summer, it might be best to just be grateful for the cucumbers you did harvest, and call it a day.

Commercial organic growers will also have a difficult time managing cucurbit downy mildew. According to N.C. State plant pathologist Lina Quesada-Ocampo, the only OMRI labeled active ingredients that have some efficacy are fixed copper formulations.

However, even without access to the more effective fungicide controls, both organic growers and homeowners can reduce the likelihood of infection by implementing certain cultural practices. These include planting early in the season to avoid the heaviest disease pressure; looking for and planting tolerant varieties; and not allowing water to remain on the foliage for extended periods of time. Regarding the latter point, many of the digital images we receive from home gardens show crowded plantings with little space in between plants, which leads to reduced air movement, easier movement of pathogens from leaf to leaf, and foliage that stays wetter for longer periods of time.

To reduce foliage wetness, plant in the sunniest locations possible, allow for some open space in between plants, and avoid overhead irrigation.

If you believe you have cucurbit downy mildew on your cucumbers, or other cucurbit crops such as watermelons, cantaloupe, squash and pumpkins, provide leaf samples or good digital images to your local Extension office. This is especially important early in the season, so that plant pathologists in North Carolina can track the disease and provide commercial growers with accurate information on the pathogen’s whereabouts.

Tom Glasgow is the Craven County Cooperative Extension director. He can be reached at teglasgo@ncsu.edu.

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